1. Field of the Invention
This invention relates to a method for processing superconducting thin films to form fine grooves, etc., in the films.
2. Description of the Related Art
Josephson junctions made of SNS (superconducting, normal conducting, superconducting) junctions or SIS (superconducting, insulator, superconducting) junctions are used with superconducting devices such as superconducting transistors and superconducting quantum interference devices. The Josephson junction can be easily formed by depositing S, N, and S layers in order, but deposition of such thin films introduces a difficult problem of installation of electrodes to the thin films. Then, fine grooves, etc., are formed in a superconducting thin film for dividing this superconducting thin film into sections, each of which is connected to an electrode. This method provides a planar type where two superconducting thin films are arranged on a single plane, thus greatly facilitating electrode installation, etc.
The ion beam sputter etching method is known as a method of forming fine grooves, etc., in thin films made of oxide superconductors such as the Bi-Sr-Ca-Cu-O systems.
FIG. 1 shows a processing method with a focused ion beam (FIB) as one example of the ion beam sputter etching by which an ion beam 33 having energy of several 10s to several 100s keV and focused to 1-0.1 .mu.m or so is irradiated to a superconducting thin film 32 on an MgO substrate 31 for sputtering to form a groove 35.
FIG. 2 shows a superconducting thin film processing method as another example of the ion beam sputter etching, (see H. Tsuge et al., Jpn. J. Appl. Phys. Vol. 27 No. 11 November 1988 pp. L2237-L2239, for example.), by which first a desired pattern is formed by means of resist 36 deposited on a superconducting thin film 32 (FIG. 2A), then an ion beam 37 of Ar, etc., is irradiated to the entire thin film 32 for sputtering portions not covered by the resist 36 to form grooves 35 (FIG. 2B), then the resist 36 is removed (FIG. 2C).
On the other hand, the present applicant has previously proposed an ion bombardment enhanced etching method using an alkaline aqueous solution shown in FIG. 3 (U.S. Ser. No. 07/722,009), whereby a damaged layer 34 formed in a superconducting thin film 32 on a substrate 31 by irradiation with an ion beam 33 (FIG. 3A) is removed by alkaline solution treatment to form a groove 35 (FIG. 3B).
However, the conventional processing methods described above introduce the following problems:
In the ion beam sputter etching technique, first, constituent atoms of a superconducting thin film are etched by irradiation with an ion beam, thus causing damaged layers, whose crystal structure is disarranged, to be formed on the sides of the resultant groove. The presence of the damaged layers makes the effective gap width wider than the width of the groove. Although the crystal structure of the damaged layer can be restored to some degree by proper annealing, the groove form changes, thus particularly when fine grooves are formed, there is a possibility that a disadvantage such as the grooves being in contact with each other will occur.
On the other hand, in the ion bombardment enhanced etching technique using an alkaline aqueous solution, first, a large ion dose of 4.6.times.10.sup.16 to 2.6.times.10.sup.17 ions/cm.sup.2 is required when 200-keV Si.sup.++ ions are used, for example.
Second, a damaged layer formed by irradiation with ions is removed to some degree by alkaline solution treatment, but is insufficiently removed. Thus, when a fine groove is to be formed, the damaged layers will make the effective gap width wider than the width of the groove. Although the crystal structure of the damaged layer can be restored to some degree by proper annealing, the groove form changes, thus particularly when fine grooves are formed, there is a possibility that a disadvantage such as the grooves being in contact with each other will occur.
Third, when a superconducting thin film is treated by using an alkaline aqueous solution, although the superconducting transition temperature does not change, the electric resistance at room temperature gradually increases with the treatment time as shown in FIG. 4; the alkali treatment has some effect on the superconducting thin film.
Fourth, when a superconducting thin film is treated by using an alkaline aqueous solution, an exposed substrate is exposed to the alkaline aqueous solution and pure water for washing after the treatment; some of the MgO used as the substrate is dissolved in the water.